Insulated wire

ABSTRACT

An insulated wire having a silane-modified polyamideimide resin, obtainable by reacting an alkoxy- or aryloxy-silane partial condensate containing a glycidyl ether group, with a polyamideimide resin having a carboxyl group and/or an acid anhydride group at an end thereof, that is coated and baked on a conductor directly or through another insulating layer.

TECHNICAL FIELD

The present invention relates to an insulated wire preferable to be usedin a coil for, for example, a motor and a transformer, which insulatedwire has good insulating property and excellent resistance to working.

BACKGROUND ART

Insulated wires coated with an electrical insulator are used in a largeamount in many applications as coils incorporated into a variety ofelectric machinery and tools. Many insulated wires are used especiallyin electric machinery and tools, represented by motors and transformers.In recent years, progress has been made in both miniaturization and thelevel of performance characteristics of these machinery and tools, andnow such insulated wires are often used by bundling them into a verynarrow space. Specifically, it is no exaggeration to say that theperformance of a rotator, such as a motor, is determined by how manywires are held in a cross section of a stator slot. As a result of this,the ratio (space factor) of the total sectional area of each conductor(the total sectional area of conductor in each wire) to the sectionalarea of the stator slot has been highly increased in recent years.

When insulated wires whose cross section has a round shape are closelypacked inside of a stator slot, voids forming dead space, or the largesectional area of an insulating film, may be a barrier to increase thespace factor. For avoiding this, when insulated wires are subjected tocoil-winding, in order to improve the space factor as much as possible,the wires are pushed into the stator slot to such an extent that thewires having a cross section of a round shape are deformed. However, asexpected, remarkable reduction in the sectional area of the insulatingfilm has not been made, because it may sacrifice electrical performance(e.g. dielectric breakdown property).

In view of the above, as means for improving the space factor, anattempt has been made, very recently, to use an insulated wire of arectangular wire with a conductor having a sectional shape similar togaquadrangle (a regular square or a rectangle). Use of the rectangularwire brings about a dramatic improvement in the space factor. However,it is difficult to apply an insulating film uniformly on a rectangularconductor, and it is particularly difficult to control the thickness ofthe insulating film in the case of insulated wire having a smallsectional area. Therefore, the rectangular wire has not been widely usedso much.

With regard to rectangular windings, there are several proposalsconcerning the method of manufacturing the windings and insulatingmaterials of the windings. For example, a method of adhering apolyesterimide film uniformly onto a rectangular conductor is describedin JP-A-2000-260233 (“JP-A” means unexamined published Japanese patentapplication), proposing an approach from the material side.

As to conventional insulated wires using a round a conductor, manyattempts are being made to improve the insulating performance of thefilm. For example, the use of resins, such as polytetrafluoroethylene,having a low dielectric constant, as the insulating film, is alreadyknown. However, these resins having a low dielectric constant have notbeen used in the field of enameled wires, because these resins having alow dielectric constant are thermoplastic, and also it is difficult toform such a thin film, as in the case of enameled wires.

Similarly, a method in which fine particles of metal oxides (e.g.titanium oxide and silica) are added in a varnish, to form an insulatingfilm, is conventionally adopted. As a result, although an improvement indielectric breakdown voltage is not observed, the occurrence of a coronain a high frequency region (for example, 1 kHz or more) is reduced, asis known. It is estimated that the reason dielectric breakdown voltageis not improved, though the occurrence of a corona is suppressed, isthat, when the fine particle of the metal oxide is added to the varnish,air components, such as oxygen, are caught in the surface of the fineparticle of the metal oxide, and the involved parts of the surface areconverted into dielectrics, and therefore no improvement in dielectricbreakdown voltage is observed.

The characteristics of the insulating film that are required when thecoiling of a motor or transformer is conducted, include resistance toworking of the insulating film. This is because, in the aforementionedcoiling, the electrical insulating property is reduced if the wire filmis damaged.

Various methods are proposed to provide the wire film with resistance toworking. For example, these methods include one in which lubricity isimparted to an insulating film, to lower the friction coefficient,thereby decreasing external damage during coiling, and a method in whichadhesion between an insulating film and an electric conductor isimproved, to prevent the film from peeling from the conductor, therebymaintaining the electrical insulating property that the insulating filmoriginally has.

As the former method of imparting lubricating ability, a method in whichthe surface of a wire is coated with a lubricant, such as wax, or amethod in which a lubricant is added in an insulating film and then isallowed to breed out on the wire surface when a wire is produced,thereby imparting lubricating ability, is conventionally adopted andpractically used in many examples. However, this method of providinglubricating ability does not improve the mechanical strength of the wirefilm itself, in any sense. Therefore, this method appears to have aneffect on factors of external damage; however, there is a limitation onthese effects, in actuality.

Various methods have been hitherto proposed concerning improving theadhesion between the conductor and the insulating film. As specificexamples of an insulating coating to be used for this purpose, 1) aheat-resistant coating, composed of a polyamideimide resin, analkoxy-modified amino resin, and benzotriazole (JP-A-3-37283), and 2) acoating composed of a polyamideimide resin and trialkylamine(JP-A-6-111632), are proposed. Wires produced by these measures arefound to have an effect to pass the reciprocating abrasion test (test inwhich a relatively low load is applied to the wire to be tested and thefilm is rubbed by a bead-needle). However, in the one-way abrasion test(the test prescribed in JIS C 3003; the film is scratched by a pianowire while applying a load gradually to the wire), a satisfactory effectis not observed. In recent years, the latter test method is regarded asimportant in testing for film damage. Further, many wires produced usingonly means of improving adhesion are decreased in the reciprocatingabrasion value with a decrease in the thickness of the film, and theadhesion of these wires is eventually decreased to the same level asthat of the conventional wire in which no means of improving adhesion isutilized.

On the other hand, a method is proposed in which many rigid structuresare introduced into a molecule, in light of the molecular structure ofthe resin, to improve the film strength, thereby decreasing workingdamage to the film. JP-A-6-196025 describes such an insulated wirehaving an insulating film having a prescribed tensile strength andtensile elasticity modulus. Such an insulated wire is found to have anoutstanding effect in the one-way abrasion test, and it ensures thatdamage to the film when coiling can be prevented, even if the film isdecreased in thickness. However, such an insulated wire is more reducedin the level of flexibility after it is elongated or is subjected tothermal history, compared with the conventional wire, and it might crackand break down in the film because of insufficient flexibility when itis subjected to bending under particularly severe conditions.

DISCLOSURE OF THE INVENTION

The inventors of the present invention have found, as a result ofstudying the reasons to cause dielectric breakdown in an insulating filmof conventional insulated wires, that a material that is resistantagainst dielectric breakdown, that is, an insulating film having aspecific silane-modified polyamideimide resin coated and baked thereon,as an insulating film material that has a low solid dielectric constantand a high dielectric breakdown voltage, significantly contributes toimproving the insulation property and resistance to working of aninsulated wire having such an insulating film. The present invention hasbeen completed based on this discovery.

The present invention is an insulated wire that comprises asilane-modified polyamideimide resin, obtainable by reacting an alkoxy-or aryloxy-silane partial condensate containing a glycidyl ether group,with a polyamideimide resin having a carboxyl group and/or an acidanhydride group at an end thereof, that is coated and baked on aconductor directly or through another insulating layer.

Further, the present invention is an insulated wire that comprises asilane-modified polyamideimide resin having a silicon content of 1 to 15mass %, that is coated and baked on a conductor directly or throughanother insulating layer.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be hereinafter explained in detail.

In the present invention, there is no particular limitation on apolyamideimide resin to be used as a base resin of the silane-modifiedpolyamideimide resin that is utilized to form the insulating film, ifthe polyamideimide resin has a carboxyl group and/or an acid anhydridegroup at an end thereof. As the polyamideimide resin, use can be made,for example, of a resin obtained by directly reacting a tricarboxylicacid anhydride with a diisocyanate in a polar solvent, or a resinobtained by reacting a tricarboxylic acid anhydride with a diamine in apolar solvent to introduce an imide bond first and then by amidating theresulting product by using a diisocyanate, in a usual manner.

As the tricarboxylic acid anhydride that can be used for preparing thispolyamideimide resin, generally, a trimellitic acid anhydride ispreferably used. In this case, a part of the amount of tricarboxylicacid anhydride may be replaced by a tetracarboxylic acid anhydride whenit is reacted. As the tetracarboxylic acid anhydride in this case, usecan be made, for example, of pyromellitic acid dianhydride,3,3′,4,4′-benzophenonetetracarboxylic to acid dianhydride, or the like.Further, a part of the amount of tricarboxylic acid anhydride may bereplaced by another acid or acid anhydride, for example, trimelliticacid, isophthalic acid or terephthalic acid. On the other hand, theexamples of the diisocyanate that can be reacted with the tricarboxylicacid anhydride, include aromatic diisocyanates such as4,4-diphenylmethane diisocyanate and tolylene diisocyanate, and theexamples of the diamine include aromatic diamines such asm-phenylenediamine, 4,4′-diaminodiphenyl ether,4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenylsulfon and4,4′-diaminobenzophenone. Further, for the imidation,N,N′-dimethylformamide may be used. Further, examples of the polarsolvent include N-methyl-2-pyrrolidone, N,N′-dimethylformamide,dimethylacetoamide, and the like. Among them, N-methyl-2-pyrrolidone canbe preferably used.

To the thus-obtained polyamideimide resin (a base resin) solution, isadded an alkoxy- or aryloxy-silane partial condensate containing aglycidyl ether, group, which is a product of dealcoholization reactionof glycidol with an alkoxy- or aryloxy-silane partial condensate, andthe polyamideimide resin is reacted with the alkoxy- or aryloxy-silanepartial condensate containing a glycidyl ether group, thereby asilane-modified polyamideimide resin can be obtained. As a typicalexample of a method of preparing the silane-modified polyamideimideresin, the method described below can be mentioned.

Any commonly known alkoxy- or aryloxysilane partial condensate(condensation product) may be employed in the present invention. Thealkoxy- or aryloxysilane partial condensate for use in the presentinvention may be, for example, a linear condensate (e.g.methoxyorganosiloxane), produced as a result of condensation bydealkyletherization reaction of at least two molecules of alkoxy- oraryloxysilane compounds. Herein, the alkoxy- or aryoxysilane compoundsto be condensed may be the same or different from each other. The alkoxygroup of the alkoxysilane partial condensate is preferably an alkoxygroup having 1 to 6 carbon atoms, and more preferably an alkoxy grouphaving 1 to 4 carbon atoms. Preferable examples thereof include methoxy,ethoxy, propoxy, butyloxy, and the like. The aryloxy group is preferablyan aryloxy group having 6 to 10 carbon atoms, and more preferably anaryloxy group having 6 to 8 carbon atoms. Preferable examples thereofinclude phenyloxy, dimethylphenyloxy, methylphenyloxy, and the like. Theaforementioned alkoxy group and aryloxy group may further have asubstituent thereon. Specific examples thereof include condensates, suchas methoxypolyorganosiloxane, ethoxypolyorganosiloxane, andphenyloxypolyorganosiloxane. In addition to the above, copolymers ofphenyloxyorganosiloxane and methoxyorganosiloxane or the like may beused. Typical examples of commercially-available products of themethoxyorganosiloxane that is a partial condensate of tetramethoxysilaneinclude “M SILICATE 51” (trade name) manufactured by Tama Kagaku Kogyo.

The average number of the silicon (Si) in one molecule of the alkoxy- oraryloxysilane partial condensate is preferably 2 to 100. When theaverage number of the Si is less than 2, the amount of alkoxy- oraryloxysilane, which is not reacted but is distilled off from the systemtogether with an alcohol that is used as a solvent, is increased toomuch, when it is reacted with glycidol, in some cases. If the averagenumber of the Si is more than 100, reactivity with glycidol becomes poorand it is difficult to obtain the glycidyl ether group-containingalkoxy- or aryloxysilane partial condensate to be intended, in somecases. The average number of the Si per molecule is more preferably 3 to20, taking the materials availability into consideration.

For example, the following method may be used for the dealcoholizationreaction between each of these alkoxy- or aryloxysilane partialcondensate and glycidol. To 1 mol of the tetramethoxysilane partialcondensate (the average number of the Si per molecule: 4), is added 2mol of glycidol, and the resultant mixture is heated to about 120° C. ina bulk condition, thereby the generation of an alcohol can be observed.The reaction is continued while this alcohol is distilled off from thesystem, thereby methoxytetraorganosiloxane diglycidyl ether that is theglycidyl ether group-containing alkoxy- or aryloxysilane partialcondensate can be obtained. The glycidyl ether group-containing alkoxy-or aryloxysilane partial condensate produced in this reaction containstwo grycidyl groups in one molecule. At this time, it is preferable touse an organic tin-series catalyst as a reaction catalyst, since thereaction proceeds more rapidly.

In order to react the resulting glycidyl ether group-containing alkoxy-or aryloxysilane partial condensate with the polyamideimide resin, forexample, the following method can be utilized. 200 g of a solutionobtained by dissolving 25% by mass of the polyamideimide resin inN-methyl-2-pyrrolidone and 5.17 g ofmethoxytetraorganosiloxane-diglycidyl ether prepared by theaforementioned method are heated to 95° C., while mixing the both in anappropriate heatable container. The resultant mixture was reacted at 95°C. for 4 hours, and then 5.17 g of N-methyl-2-pyrrolidone was added tothe reaction mixture, followed by cooling, to obtain a silane-modifiedpolyamideimide resin solution having 25% of a nonvolatile component. Inthis case, the content of silicon is 4.29% by mass.

The content of silicon in the silane-modified polyamideimide resin foruse in the present invention is preferably in the range of 1 to 15 mass%. If the content of silicon is too small, the effect achieved byinsulated wires having the silane-modified polyamideimide resin coatedand baked thereon (the effect of improving scratch-resistance propertyof the wires, in particular) is hardly obtained, resulting in only thesame performance as the conventional insulating film on the other hand,if the content of silicon is too large, the appearance of the resultantinsulated wire after baking is not so good, and minute roughnessoccurred on the surface of the insulated wire, which may have anaffection on the electric property of the wire. Thus, the content ofsilicon is to be determined appropriately, in consideration of theaforementioned facts.

This content of silicon is almost determined by the molar ratio adoptedwhen the resin is synthesized. The content is determined more exactly byusing the resonance spectrum of ²⁹Si measured using a solid NMR. In thiscase, it is general to use polydimethylsiloxane (−34 ppm) as a standardsample.

The insulated wire of the present invention can be produced according toa method in which the silane-modified polyamideimide resin is coatedonto a conductor as it is, followed by baking, or a method in which thesilane-modified polyamideimide resin is coated onto a conductor throughanother insulating material, followed by baking. Further, for example,the silane-modified polyamideimide resin may also be used as aninsulating material in an intermediate layer. In this case, it ispreferable that a known polyamideimide resin is coated to provide atleast one layer as an under layer, then the silane-modifiedpolyamideimide resin is coated thereon to provide one layer, and furthera polyamideimide resin is coated to provide an upper layer thereof.

When the silane-modified polyamideimide resin is coated on a conductorthrough another insulating material, there is no particular limitationon the another insulating material, and any one of insulating materialsusually used in insulated wires may be used. As examples of thematerial, polyesters, heat-resistant modified polyesters, polyurethanes,polyesterimides, polyamideimides, polyimides, and the like can beexamplified.

Moreover, when the silane-modified polyamideimide resin is coated on aconductor, it is possible to make the resin itself possessself-lubricity. As a method for the self-lubricity, a known method maybe used, for example, a method in which polyethylene wax is added in aresin solution is most usual.

In the present invention, there is no particular limitation on thethickness of the silane-modified polyamideimide resin layer, althoughthe thickness thereof differs depending on, for example, whether theresin layer is solely formed directly on the conductor, or whether theresin layer is provided through another insulation layer. In general,the thickness of the silane-modified polyamideimide resin layer ispreferably in the range of 0.001 to 0.040 mm, and more preferably in therange of 0.002 to 0.012 mm.

The baking after coating of the silane-modified polyamideimide resin maybe carried out in a manner similar to that of the conventionalcoating-and-baking process. The baking temperature is generally in therange of 400 to 550° C., and preferably in the range of 480 to 530° C.Further, the coating-and-baking process of the silane-modifiedpolyamideimide resin is preferably conducted as a process in whichcoating and baking are repeated a plurality of times, such that theresin is baked, after each coating, with a baking time of generally 15seconds to 1 minute, preferably 20 to 25 seconds, and thiscoating-and-baking process is repeated generally at least 6 times,preferably 15 times or more. In such a process in which coating andbaking are repeated a plurality of times, the total time of the coatingand baking is generally 1 minute and 30 seconds to 15 minutes.

The silane-modified polyamideimide resin itself to be used in thepresent invention has been developed by Arakawa Kagaku Kogyo Co., and itis synthesized according to a method developed by the same company.

In the present invention, by coating and baking the silane-modifiedpolyamideimide resin on a conductor, silica (SiO₂) portions formed bythe alkoxysilyl group or aryloxysilyl group of the resin, i.e., ahigher-order network structure of siloxane bonding, is formed in theresulting insulating film. It is assumed that, due to the silicaportions described above, the thus-obtained insulating film has a lowdielectric constant.

The insulated wire of the present invention has high resistance toworking, and it hardly causes poor insulation, since the breakdown ofthe insulating film does not occur and cracks of the film are hardlyoccurred even if a high load is applied under the severe conditions forcoil-winding. Further, the insulated wire of the present invention has ahigh dielectric breakdown voltage, and it has the excellent insulationcharacteristics that even if the thickness of the film is made thin, nodielectric breakdown takes place. Further, the insulating film for usein the insulated wire of the present invention exhibits a low dielectricconstant, which is as low as that of a polyimide. According to theabove, when the insulated wire of the present invention is used, forexample, in a transformer or a motor, it can be used in the condition ofa high space factor, and it hardly causes poor insulation under such acircumstance. Therefore, according to the insulated wire of the presentinvention, a highly reliable coil can be provided, thereby exhibitingsuch excellent effects to contribute to miniaturization, cost reductionand improvement in reliability of the entire machinery and tools inwhich the coil utilizing the insulated wire of the present invention isused.

EXAMPLE

The present invention will be hereinafter explained in more detail basedon the following examples, but the invention is not limited to these.

(Preparation of a Polyamideimide Resin)

A four neck flask with a volume of 2 little was equipped with a stirrer,a cooling tube and a calcium chloride tube, and the flask was chargedwith 192 g (1 mol) of trimellitic acid anhydride, 250 g (1 mol) of4,4′-diphenylmethane diisocyanate and 663 g of N-methyl-2-pyrrolidone.The resultant mixture was reacted for 2 hours at 80° C. and for 5 hoursat an elevated temperature of 140° C. Thereafter, the reaction mixturewas cooled to 50° C., and 163 g of N,N′-dimethylformamide was added tothe reaction mixture. According to the above procedure, a polyamideimideresin solution having a resin concentration of 30% was obtained.

(Preparation of a Glycidyl Ether Group-Containing Alkoxysilane PartialCondensate (1))

A four-neck flask with a volume of 1 little was equipped with a stirrer,a cooling tube and a condenser tube, and the flask was charged with148.16 g (2 mol) of glycidol and 474.10 g (1 mol in terms of an averagemolecular weight) of a tetramethoxysilane partial condensate (theaverage number of the Si: 4). The resultant mixture was heated to 90° C.with stirring under a nitrogen stream. After reached 90° C. by heating,0.70 g of dibutyltin dilaurate was added thereto as a catalyst, and theresultant mixture was reacted as it was. Methanol generated as abyproduct in the reaction was removed using a distilling column. Whenthe amount of distilled-off methanol reached 50 g, the reaction mixturewas cooled to room temperature. The reaction time during this processwas 4 hours at 90° C. After the reaction mixture was cooled to roomtemperature, methanol left unremoved was removed under reduced pressure,to find that the total distilled-off amount of methanol was 64.0 g. As aresult, 558.26 g of a glycidyl ether group-containing alkoxysilanepartial condensate (a) was obtained. The ratio of “the average number ofthe Si per one molecule of the product/the average number of theoxysilane ring (the glycidyloxy group on Si) per one molecule of theproduct” of this condensate (a) was 2.

(Preparation of a Glycidyl Ether Group-Containing Alkoxysilane PartialCondensate (2))

In the same manner as in the above (1), a flask was charged with 74.08 g(1 mol) of glycidol and 369.07 g (⅓mol) of a tetramethoxysilane partialcondensate (the average number of the Si: 10), and the resultant mixturewas heated to 90° C. with stirring under a nitrogen stream. Afterreached 93° C. by heating, 0.70 g of dibutyltin dilaurate was addedthereto, as a catalyst, and the resultant mixture was reacted as it was.Methanol generated as a byproduct in the reaction was removed using adistilling column. When the amount of distilled-off methanol reached 20g, the reaction mixture was cooled to room temperature. The reactiontime during this process was 6 hours at 90° C. After the reactionmixture was cooled to room temperature, methanol left unremoved wasremoved under reduced pressure, to find that the total distilled-offamount of methanol was 32.0 g. As a result, 411.15 g of a glycidyl ethergroup-containing alkoxysilane partial condensate (b) was obtained. Theratio of “the average number of the Si per one molecule of theproduct/the average number of the oxysilane ring per one molecule of theproduct” of this condensate (b) was 3.

TABLE 1 Condensate (a) Condensate (b) Average number of the Si in the 410 tetramethoxysilane partial condensate Average number of the Si perone 2  3 molecule of the product/average number of the oxysilane ringper one molecule of the product

(Preparation of a Silane-Modified Polyamideimide Resin (1))

A four neck flask with a volume of 1 little was equipped with a stirrerand a cooling tube, and the flask was charged with 50 g of thepolyamideimide resin solution prepared as in the paragraph of thePreparation of a polyamideimide resin. Thereto, was added 5.17 g of theglycidyl ether group-containing alkoxysilane partial condensate (a)prepared as in the Preparation of a glycidyl ether group-containingalkoxysilane partial condensate (1). The resultant mixture was stirredat 95° C. for 4 hours. Thus, a silane-modified polyamideimide resin (thecontent of silicon: 4.29%) (AI-1) was obtained.

(Preparation of a Silane-Modified Polyamideimide Resin (2))

A four neck flask with a volume of 1 little was equipped with a stirrerand a cooling tube, and the flask was charged with 500 g of thepolyamideimide resin solution prepared as in the paragraph of thePreparation of a polyamideimide resin. Thereto, was added 2.58 g of theglycidyl ether group-containing alkoxysilane partial condensate (a)prepared as in the Preparation of a glycidyl ether group-containingalkoxysilane partial condensate (1). The resultant mixture was stirredat 95° C. for 4 hours. Thus, a silane-modified polyamideimide resin (thecontent of silicon: 2.15%) (AI-2) was obtained. The content values ofsilicon shown in Table 2 and Table 3 were obtained by calculation fromthe resin mole ratios of siloxane and polyamideimide at the time ofcharging these substances.

TABLE 2 AI-1 AI-2 Polyamideimide resin solution (g) 500 500 Type ofglycidyl ether group- Condensate Condensate containing alkoxysilane (a)(a) partial condensate (ditto) Amount (g) 5.17 2.58 Degree ot silanemodification 4.29 2.15 of the polyamideimide resin (content of silicon:mass %)

(Preparation of Silane-modified Polyamideimide Resins (3 to 6))

A four neck flask with a volume of 1 little was equipped with a stirrerand a cooling tube, and the flask was charged with 500 g of thepolyamideimide resin solution prepared as in the paragraph of thePreparation of a polyamideimide resin. Thereto, was added the glycidylether group-containing alkoxysilane partial condensate (b) prepared asin the Preparation of a glycidyl ether group-containing alkoxysilanepartial condensate (2). The resultant mixture was stirred at 95° C. for4 hours. Thus, silane-modified polyamideimide resins (AI-3 to AI-6) wereobtained.

TABLE 3 AI-3 AI-4 AI-5 AI-6 Polyamideimide 500 500 500 500 resinsolution (g) Type of glycidyl Conden- Conden- Conden- Conden- ethergroup- sate sate sate sate containing (b) (b) (b) (b) alkoxysilanepartial condensate (ditto) Amount (g) 1.20 1.42 3.07 16.1 Degree ofsilane 0.75 1.18 2.56 13.4 modification of the polyamideimide resin(content of silicon: mass %)

Examples 1 to 7 Comparative Example 1

Each of the silane-modified polyamideimide resins prepared in the abovemanner was coated onto a conductor, followed by baking, to obtain adesired insulated wire, respectively. The insulated wire was formedaccording to the following manner.

As the conductor of the insulated wire, a copper wire having a diameterof 1.0 mm was used. With regard to the baking of the resin, the resinwas coated and baked plural times, using an air-heating circular-typebaking furnace having a furnace length of 7 m and an atmospherictemperature of 500° C., to form an insulating film having apredetermined film thickness. As the insulating film, an insulatinglayer of a single layer was provided in each of Examples 1 to 5 andComparative Example 1. On the other hand, two insulating layers composedof an upper layer and a lower layer were provided in each of Examples 6and 7. The thickness of each of these upper and lower insulating layersis described as the ratio of thickness in the parenthesis following thefilm thickness in Table 5. The silane-modified polyamideimide resincoatings (AI-1 to AI-6) that were prepared in the above were used forforming insulating layers. Further, as the insulation coatings to beused for comparison, HI-406 and HI-406A (trade names) manufactured byHitachi Chemical were respectively used, as polyamideimide resins.

Example 8

A flask equipped with an optional stirrer was charged with 27 g ofxylene and 3 g of polyethylene wax (Polyethylene Wax 400P, trade name,manufactured by Mitsui Chemical), and the resultant mixture was stirredat 120° C. for one hour. After the resultant mixture solution becametransparent and uniform, it was cooled rapidly with stirring, to preparea polyethylene wax dispersion (30 g).

This polyethylene wax dispersion (30 g) was added to 500 g of thesilane-modified polyamideimide resin (AI-3) that was preparedpreviously, and the resultant mixture was stirred sufficiently such thatthe polyethylene wax was dispersed in the polyamideimide resin, toproduce self-lubricating varnish.

This self-lubricating varnish was used for forming an insulated wire inthe same manner as in Example 6.

The damage resistance of the resulting insulated wires in theaforementioned Examples 1 to 8 and Comparative Example 1 were evaluated,according to the test methods as shown below.

(One-way abrasion test): The test described in Item 10 of JIS C3003 wascarried out. The result is expressed with N unit. It is shown that thehigher the value of the result is, the more difficult the peeling of thefilm is caused.

(Impact drop test): The insulated wire to be tested was fixed onto aV-groove on a metal plate, in which the V-groove provided on the metalplate surface was shallower than the diameter of the insulated wire.Using a knife-edge (a point angle of 55 degrees and a tip curvaturer=0.5) that was arranged so as to form a right angle with thelongitudinal direction of the insulated wire, an a impact load of 100 g,500 g or 1000 g as the total load was, respectively, dropped on theinsulated wire, at an angle of 45 degrees with a horizontal plane for adistance of 370 mm as the dropped distance of the load (actual movinglength of the load). Then, the state whether the insulating film of thewire was broken or not, was evaluated in a test on leakage currentcaused by damaged portions of the wire. The leakage current test wascarried out according to the pin hole test method described in JISC3003, except that the polarity of each of the positive and negativeelectrodes were inverted, using an ammeter for detection. Using theconductor as the positive electrode and the water side as the negativeelectrode, and a voltage of 12 V was applied between the electrodes, toread the value of leakage current from the ammeter. The value shows thatthe larger the value is, the more easily the film is damaged.

The resistance to external damages was judged from the results of bothof the impact drop test and the one-way abrasion test.

The results are shown in Tables 4 and 5.

TABLE 4 Example Example Example Example Example 1 2 3 4 5 Insulatingcoating 1 AI-1 AI-2 AI-4 AI-5 AI-6 (upper layer) Ratio of the filmInsulating coating 2 — — — — — (lower layer) Ratio of the film Surfacetreatment of Paraffin wax coating the wire Finish diameter (mm) 1.0721.072 1.072 1.072 1.072 Film thickness (mm) (Upper layer/lower 0.0360.036 0.036 0.036 0.036 layer) ratio One-way abrasion (N) 26 26 27 27 27Impact drop test Leakage current Load  100 g 0.00 0.00 0.00 0.00 0.00Load  500 g 0.00 0.00 0.00 0.00 0.00 Load 1000 g 0.11 0.08 0.03 0.030.03

TABLE 5 Comparative example 1 Example 6 Example 7 Example 8 Insulatingcoating 1 HI-406*^(l) AI-2 AI-2 AI-3 (upper layer) (30%) (30%)Polyethylene Ratio of the film wax (20%) Insutating coating 2 — HI-406HI- HI-406A (lower layer) (70%) 406A*² (80%) Ratio of the film (70%)Surface treatment of the wire Finish diameter 1.072 1.072 1.072 1 .072(mm) Film thickness (mm) 0.036 0.036 0.036 0.036 (Upper layer/lower(30/70) (30/70) (20/80) layer) ratio One-way abrasion 18 27 27 30 (N)Impact drop test Leakage current Load  100 g 0.05 0.00 0.00 0.00 Load 500 g 0.20 0.00 0.00 0.00 Load 1000 g 1.50 0.00 0.03 0.03 (Note)*¹HI-406: A polyamideimide resin coating, trade name, manufactured byHitachi Chemical. *²HI-406A: A polyamideimide resin coating, trade name,manufactured by Hitachi Chemical (improved in adhesion to a conductor).

In the impact drop test, each of the Examples according to the presentinvention showed very high film rupture strength.

Contrary to the above, in the case of the wire of Comparative Example 1,it may be estimated that since this wire did not have thesilane-modified polyamideimide resin layer, which is defined in thepresent invention, the wire for comparison could not disperse the forcewhen a high load was applied and damages progressed to the conductor ata stroke, with the result that the intended film rupture strength, whichis one index on resistance to working, could not be obtained.

For the evaluation of the insulating property, the electrical insulatingproperty of each of insulated wires obtained in the above Examples 1 to5 and Comparative Example 1 was evaluated, according to the followingtest methods.

(Dielectric breakdown voltage): The test described in JIS C3003 wascarried out. The results are shown with kV unit, showing that the higherthe value is, the higher the dielectric breakdown voltage is. Further,the relationship between the dielectric breakdown voltage and thethickness of the formed insulating film is also shown, in terms of theratio of the two factors (voltage to thickness).

(Dielectric constant): The dielectric constant of the formedinsulating-film was measured. The dielectric constant was measured usingan LCR meter in which measuring frequency was set to 1 kHz. Further,measuring temperature was set to room temperature (25° C.) and 100° C.

The results are shown in Table 6.

TABLE 6 Example Example Example Example Example Comparative 1 2 3 4 5example 1 Insulating coating AI-1 AI-2 AI-4 AI-5 AI-6 HI-406* Surfacetreatment of Paraffin wax coating the wire Finish diameter (mm) 1.0721.072 1.072 1.072 1.072 1.072 Film thickness (μm) 36 36 36 36 36 36Dielectric breakdown voltage (kV) <Minimum> 13.7 13.5 12.8 13.0 13.711.3 <Average> 14.4 15.5 14.9 15.1 15.3 13.1 Dielectric breakdown 400431 414 419 425 363 voltage/film thickness (V/μm) Dielectric constant3.6 3.5 3.6 3.5 3.4 4.2 (room temperature: 25° C.) 1 MHZ Dielectricconstant 3.7 3.8 3.8 3.7 3.8 4.3 (100° C.) 1 MHZ (Note) *1 HI-406: Apolyamideimide resin coating, trade name manufactured by HitachiChemical.

Industrial Applicability

The insulated wire of the present invention is excellent in insulationproperty and resistance to working. In particular, the insulated wire ofthe present invention exhibits an excellent insulation property, whichis comparable to or exceeds the insulation property of the conventionalthick insulating film, even if the insulating film of the wire of thepresent invention is thin. Accordingly, the insulated wire of thepresent invention is preferable for use in a coil of, for example, amotor or a transformer.

Having described our invention as related to the present embodiments, itis our intention that the invention not be limited by any of the detailsof the description, unless otherwise specified, but rather be construedbroadly within its spirit and scope as set out in the accompanyingclaims.

What is claimed is:
 1. An insulated wire, comprising a silane-modifiedpolyamideimide resin, obtainable by reacting an alkoxy- oraryloxy-silane partial condensate containing a glycidyl ether group,with a polyamideimide resin having a carboxyl group, an acid anhydridegroup, or both a carboxyl group and an acid anhydride group at an endthereof, that is coated and baked on a conductor directly or throughanother layer.
 2. The insulated wire according to claim 1, wherein saidalkoxy- or aryloxy-silane partial condensate containing a glycidyl ethergroup is obtainable by a dealcoholization reaction of a glycidol with analkoxy- or aryloxy-silane partial condensate.
 3. The insulated wireaccording to claim 1 or 2, wherein the silane-modified polyamideimideresin has a silicon content of 1 to 15 mass %.
 4. The insulated wireaccording to any one of claim 1 or 2, wherein the silane-modifiedpolyamideimide resin is being baked at a baking temperature of 400 to550° C. after coating.
 5. The insulated wire according to any one ofclaim 1 or 2, wherein the silane-modified polyamideimide resin is beingcoated and baked in process in which coating and baking are repeated aplurality of times, such that a step of baking the resin, after eachcoating, with a baking time of 15 seconds to 1 minute, is repeated atleast 6 times.
 6. The insulated wire according to any one of claim 1 or2, wherein the thickness of the insulating film provided by coating andbaking of the silane-modified polyamideimide resin, is 0.001 to 0.040mm.
 7. The insulated wire according to any one of claim 1 or 2, whereina mixture obtained by adding polyethylene wax to a solution of thesilane-modified polyamideimide resin is being coated and baked, as thesilane-modified polyamideimide resin, on a conductor directly or throughanother insulting layer, thereby a film of the silane-modifiedpolyamideimide resin having self-lubricity property is provided.
 8. Theinsulated wire according to any one of claim 1 or 2, wherein at leastone layer of a polyamideimide resin layer is being provided by coating,as a lower layer, one layer of the silane-modified polyamideimide resinlayer is being provided by coating, as an intermediate layer, on thelower layer; and a polyamideimide resin layer is being provided bycoating, as an upper layer, on the intermediate layer.
 9. An insulatedwire, comprising a silane-modified polyamideimide resin having a siliconcontent of 1 to 15 mass %, that is coated and baked on a conductordirectly or through another insulating layer.